Many wireless communication techniques and components require knowledge of channel state to achieve their optimal performances. In practice, this knowledge is often acquired by estimation. The estimation can be performed blindly by using unknown data symbols only [1]. More frequently, it is performed with the aid of some known symbols. If the channel is fading slowly enough such that the channel parameters remain approximately constant over a relatively long period of time, a training sequence can be used since the channel parameters estimated during the training period are valid for the following data transmission as well. In some practical wireless communication systems, this slow fading condition is not satisfied. In this case, pilot symbols that are interspersed with data symbols are used in conjunction with interpolation, and the channel parameter estimation has to be performed for each individual data symbol. Pilot symbol assisted modulation (PSAM) was proposed to detect signals in fast fading channels in [2]-[4].
A conventional PSAM (CPSAM) system is implemented by periodically inserting pilot symbols known to a receiver into a data sequence. After transmission over the fading channel with the data symbols, these pilot symbols are separated from the received signals and applied to a channel estimator. The channel estimator uses these pilot symbols to generate a channel gain estimate. The channel gain may have changed from the pilot symbol time instant to the data symbol time instant. Therefore, the channel gain is estimated using multiple pilot symbols and an interpolation filter. The interpolated channel gain estimates at the time instants of the data symbols are valid due to the time correlation of the fading. The channel gain estimate is used in a conventional coherent signal detector to make a data decision. It has been shown [2]-[4] that this detector is effective in combating fast fading.
In this conventional detector, estimation of the channel gain and detection of the data symbol have actually been split into two separate operations. One first obtains the channel gain estimate using the pilot symbols, and then uses this estimate in the coherent signal detector to make the data decision. Note that the optimality of the coherent signal detector is only valid under the assumption of perfect channel state information. Note further that the use of the CPSAM signal detector is essentially an intuitive realization of the coherent signal detector when the channel state information is not perfectly known.
Thus, there remains a need for improved PSAM techniques.